At present, as an optical recording medium, there is widely used a binary optical recording medium which is configured to record binary data by a pit formed by irradiation of a recording laser beam and enable the binary data to be reproduced based on whether the pit is present or absent. Further, in recent years, to meet the requirement of enhancing recording density of optical recording media, research has been conducted into the recording of data in high density by adjusting the focused beam diameter of a recording laser beam. On the other hand, a multi-level optical recording medium is being developed which is capable of recording one of different marks having a plurality of meanings, respectively, on one virtual recording cell, differently from the method of adjusting the focused beam diameter. In the multi-level optical recording medium, there are utilized, for example, characteristics thereof in which by switching the irradiation amount of a recording laser beam between multiple levels, a portion changed in properties (hereinafter referred to as a “recording mark”) appears in a portion of one virtual recording cell as a recording object in the optical recording medium, causing reduction of the light transmission ratio of the cell, and at the same time a proportion of the recording mark to the whole of the one virtual recording cell varies with the irradiation amount of the recording laser beam. In short, in this multi-level optical recording medium, when the reproducing laser beam is irradiated, the light reflection ratio of the reproducing laser beam is affected by the light transmission ratios of virtual recording cells formed with recording marks, so that the reproducing laser beam is reflected at multiple levels (e.g. five or more levels) of light reflection ratios. Therefore, by causing a plurality of data elements to be associated with multiple levels of light reflection ratios, respectively, one virtual recording cell stores one of the data elements. In this case, the term “light transmission ratio” is intended to mean a ratio of a laser beam having passed through the virtual recording cell to the reproducing laser beam irradiated onto the virtual recording cell, and the term “light reflection ratio” is intended to mean a ratio of a laser beam which has passed through the virtual recording cell to be reflected by a reflecting layer of the multi-level optical recording medium, and then has passed through the virtual recording cell again to be emitted out of the multi-level optical recording medium, to the reproducing laser beam irradiated onto the virtual recording cell.
In recording record data on the multi-level optical recording medium, it is necessary to perform recording such that individual data recorded at multiple levels can be reliably identified when they are reproduced. Accordingly, it is preferable that there are certain degrees of difference between the respective light reflection ratios of virtual recording cells having record data recorded thereon at multiple levels. On the other hand, materials used for the recording layer of the multi-level optical recording medium do not have respective unique degrees of change in properties with respect to the irradiation amount of the recording laser beam, but have different degrees of change in properties depending on the characteristics thereof. Therefore, the light reflection ratio of a virtual recording cell having a recording layer thereof unchanged in properties and the light reflection ratio of a virtual recording cell having a recording layer thereof most changed in properties also vary with materials for the recording layer. Therefore, prior to recording record data, it is necessary to specify in advance, for virtual recording cells in the multi-level optical recording medium, at least a light reflection ratio of virtual recording cells having a recording layer thereof most changed in properties and a light reflection ratio of virtual recording cells having a recording layer thereof least changed in properties. To this end, in recording record data on the multi-level optical recording medium at multiple levels, the irradiation amount of the recording laser beam is switched between the multiple levels, e.g. prior to recording the record data, to thereby vary the properties of virtual recording cells to the multiple levels, and the respective light reflection ratios of the virtual recording cells dependent on degrees of change in properties thereof are measured. This specifies the light reflection ratio of virtual recording cells having a recording layer thereof most changed in properties and the light reflection ratio of virtual recording cells having a recording layer thereof least changed in properties. Thereafter, the record data is recorded by controlling the light reflection ratio of a laser beam to multiple levels according to the record data such that the respective light reflection ratios of virtual recording cells are included between the two specified light reflection ratios.
On the other hand, in reproducing the record data recorded on the multi-level optical recording medium, it is possible to reproduce the record data only by identifying which of the multiple levels of light reflection ratios are exhibited by virtual recording cells having the reproducing laser beam irradiated thereon. Therefore, prior to reproducing the record data, it is necessary to identify in advance, in the multi-level optical recording medium, at least the light reflection ratio of a virtual recording cell having the recording layer thereof most changed in properties, and the light reflection ratio of a virtual recording cell having the recording layer thereof least changed in properties. Therefore, when record data recorded on the multi-level optical recording medium are reproduced, the respective light reflection ratios of a predetermined number of virtual recording cells are measured sequentially from a first virtual recording cell, e.g. prior to reproduction of the record data, thereby identifying how many levels the light reflection ratios of the virtual recording cells are defined into and how the light reflection ratios of the virtual recording cells are defined. After that, the record data are reproduced based on the respective identified levels of light reflection ratios.